A pressure swing adsorption (PSA) process provides an efficient and economical means for separating a multicomponent feed gas stream containing at least two gaseous components having different adsorption characteristics to produce a high purity product gas. The PSA process is based on the principle of selectively adsorbing impurities onto adsorbent materials at a relatively high pressure to form the high purity product gas, and desorbing the impurities from the adsorbent materials at relatively low pressure to regenerate the adsorbent materials and to form a secondary stream containing the impurities called the tail gas stream. The multicomponent feed gas is typically fed to one of a plurality of fixed-bed adsorption units. The fixed-bed adsorption units each contain layers of different adsorbent materials where the lower layer or layers are filled with weaker adsorbent materials, e.g., relatively low affinity for adsorbing a gaseous component, and the upper layer or layers are filled with stronger adsorbent materials, e.g., relatively high affinity for adsorbing a gaseous component.
The multiple fixed-bed adsorption units cooperatively operate in a staggered sequence to produce constant feed, product, and tail gas flows. Regardless of the number of fixed-bed adsorption units, the PSA process follows a five-step pressure-swing cycle including an adsorption step, a co-current depressurization step, a counter-current depressurization step, a purge step, and a repressurization step. During the adsorption step, the multicomponent feed gas enters a lower portion of the fixed-bed adsorption unit at a high-pressure, and as the feed gas rises in the unit, the impurities are adsorbed in the various layers of the adsorbent materials depending upon their respective adsorption selectivity to form the high purity product gas. That is, stronger adsorbent impurities are adsorbed in the lower layers of the fixed-bed containing the weaker adsorbent materials, and the weaker adsorbent impurities are adsorbed in the upper layers containing the stronger adsorbent materials. The co-current depressurization, counter-current depressurization and purge steps decrease the pressure in the fixed-bed adsorption unit and purge the unit with high purity gas from the product or co-current depressurization steps, respectively, to remove the impurities and regenerate the adsorption materials. The repressurization step increases the pressure in the fixed-bed adsorption unit with either feed gas or product gas in preparation for the next adsorption step.
Unfortunately, sometimes the PSA process is operated such that the high purity product gas is produced at a lower than target level of purity (e.g. product gas containing more impurities). For example, the PSA process may be designed to remove impurities from a hydrogen rich stream to produce a high purity product stream having about 99.9 mole % of hydrogen. However, if the process is not monitored or if such a high purity product gas is not necessarily needed for subsequent downstream processes, for example, the high purity product stream may have about 99.5, 98, 97, or even 95 mole % of hydrogen. Because the high purity product gas is used to regenerate the adsorbent materials contained in the various layers of the fixed-bed, a high purity product gas at lower than target levels of purity may not sufficiently regenerated the adsorbent materials during the depressurization and purge steps. If the PSA process is allowed to operate for an extended period of time under these conditions, irreversible adsorption (e.g. permanent adsorption) can occur causing a loss of activity of the adsorbent material. Specifically, some of the strongly adsorbent impurities, which by design are intended to adsorb onto the weaker adsorbent materials contained in the lower layers of the fixed-bed, may be adsorbed onto the stronger adsorbent material contained in the upper layers of the fixed-bed. When this occurs, the strongly adsorbent impurities may not sufficiently desorb from the stronger adsorbent materials during the depressurization and purge steps, causing impurity buildup on the adsorbent material and loss of adsorption activity. When the adsorbent materials lose activity, they need to be replaced, which is expensive and time consuming.
Accordingly, it is desirable to provide methods for controlling impurity buildup on adsorbent for PSA processes. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description of the Invention and the appended Claims, when taken in conjunction with the accompanying drawings and this Background of the Invention.